FIG. 2 is a sectional front view of a prior art solid electrolytic capacitor 1, and FIG. 1 is a perspective view of a capacitor element 2 (see Japanese Examined Patent Publication No. HEI4-19695 (1992)).
The solid electrolytic capacitor 1 includes an aluminum case 3 having a top opening, the capacitor element 2 contained in the case 3, and a rubber packing 30 which seals the top opening of the case 3. An upper edge portion of the case 3 is curved to fix the packing 30, and a plastic seat plate 31 is attached to the top of the case 3. Lead wires 21, 21 extend from the capacitor element 2 through the packing 30 and the seat plate 31, and then bent laterally.
As shown in FIG. 1, the capacitor element 2 includes an anode foil 4 of an aluminum foil coated with a dielectric oxide film and a cathode foil 5 of an aluminum foil, which are rolled together into a roll with a separator 6 of a dielectric material such as paper interposed therebetween and fixed by a tape 26. The capacitor element 2 further includes a solid electrolyte such as a TCNQ (7,7,8,8-tetracyanoquinodimethane) complex salt impregnated therein, or an electrically conductive polymer layer provided therein. Lead tabs 25, 25 respectively extend from the anode foil 4 and the cathode foil 5, and the lead wires 21, 21 respectively extend from the lead tabs 25, 25.
Where the electrically conductive polymer layer is formed between the foils 4 and 5, the capacitor element 2 is impregnated with a solution mixture containing n-butyl alcohol as a diluent, 3,4-ethylenedioxythiophene and iron p-toluenesulfonate, followed by thermal polymerization.
Although the solid electrolytic capacitor 1 having such a construction is widely used, there is a market demand for a capacitor having a smaller size and a greater capacitance. To this end, there has been proposed a capacitor which includes a cathode foil 5 coated with a metal nitride film as will be described below (see Japanese Unexamined Patent Publication No. 2000-114108).
An explanation will be given to the principle of the capacitance increase of the capacitor by coating the cathode foil 5 with the metal nitride film. In general, the dielectric oxide film is not intentionally formed on the cathode foil 5, but formed by natural oxidation. Therefore, the capacitance C of the capacitor is equivalent to a capacitance obtained by connecting the capacitance Ca of the anode foil 4 and the capacitance Cc of the cathode foil 5 in series, and represented by the following equation:C=Ca×Cc/(Ca+Cc)=Ca×1/(Ca/Cc+1)
That is, if the cathode foil 5 has the capacitance Cc, the capacitance C of the capacitor is smaller than the capacitance Ca of the anode foil 4.
Where a film 52 of a metal nitride such as TiN is formed on the cathode foil 5 by sputtering or vapor deposition, however, molecules of the metal nitride supposedly intrude into the oxide film 51 to contact an aluminum base of the cathode foil 5. Therefore, the base and the metal nitride are electrically connected to each other, so that the cathode foil 5 has no capacitance. Thus, the capacitance of the capacitor can be increased without size increase of the capacitor.
However, this arrangement has the following drawbacks.
When the cathode foil 5 coated with the metal nitride film 52 is rolled for production of the capacitor element 2, the film 52 is liable to be exfoliated or cracked due to a tensile force or a twist force applied to the cathode foil 5. As a result, a leak current is increased. Further, where the electrically conductive polymer layer is formed between the foils 4 and 5 by impregnating the capacitor element 2 with the solution mixture containing 3,4-ethylenedioxythiophene and iron p-toluenesulfonate, the film 52 is liable to be eroded because the iron p-toluenesulfonate solution is highly acidic. This also results in the increase in the leak current.
The cathode foil 5 coated with the metal nitride film 52 are gradually oxidized over time. As a result, the cathode foil 5 has a capacitance, whereby the capacitance of the solid electrolytic capacitor 1 is liable to be reduced.
It is therefore an object of the present invention to provide a solid electrolytic capacitor substantially free from the increase in leak current and having a greater capacitance and a lower ESR (equivalent series resistance).